The present invention relates to a molding or casting apparatus, and more particularly to a system and method for vibrating and compacting sand about a molding pattern in a molding flask.
In the evaporative pattern casting technique, sand is fluidized and compacted around an expendable pattern in order to establish a cavity for the casting process. A vibration device of some type is typically employed in this process to achieve fluidization of the sand so that the sand more easily enters internal passages A vibration device is similarly used to compact the sand, once it has reached all the interior surfaces. After compaction, molten metal is poured into a sprue and the liquid metal vaporizes the pattern and thus accurately replicates the pattern.
Conventional compaction devices have typically been in the form of a vibrating table. A box or flask containing the sand and expendable pattern is set on or clamped to the table. Alternatively, the flask has been supported in a guided but free-floating position relative to the vibrating table. Experience with such prior art vibrating devices, particularly when more than one axis of vibration is actuated, has revealed several deficiencies. One deficiency is the lack of synchronization of the plurality of eccentrically weighted rotors, which produces force vectors in an uncontrolled orientation to the desired pure vertical or horizontal motions Also, the applied horizontal force vectors have been eccentric to the center of gravity of the flask and sand resulting in turning moments which give substantially different motions to the sand at the top of the box than that experienced at the middle or bottom of the box. Both the lack of control of the force vectors and the turning moments tend to produce undesirable forces on the sand and pattern which can result in undesirable sand circulation irregularities and pattern distortion forces.
Another problem associated with prior art devices is the inconsistency among castings from identical patterns. Some of this inconsistency results from the problems mentioned above, but another contributing factor is the inconsistency in net vibrational forces on the flask during the initiation of each vibration cycle. In known systems, the synchronized motors or similar drive mechanisms, begin the vibration cycle from a "dead start". The initial forces acting on the flask, prior to all the motors reaching full speed, varies almost randomly from pattern to pattern.
One embodiment of a conventional device is disclosed in U.S. Pat. No. 4,593,739. This patent discloses a method and apparatus for packing sand around a mold pattern by vibration, in which the mold pattern is held in position in a mold flask in an unconnected relation to the mold flask. Sand is supplied to the mold flask to surround the mold pattern while the mold flask is vibrated to compact the sand around the pattern. The mold flask is shaken by an arrangement including a vibrator motor mounted on a compaction table and a connecting arm intermediate the table and the flask such that only horizontal forces are transmitted from the vibrator to the flask. Preferably, the connecting arm is situated such that the horizontal vibrational forces are directed in a horizontal plane extending approximately through the combined center of gravity of the mold flask and the sand.
A similar embodiment is disclosed in U.S. Pat. No. 4,600,046, wherein a molding apparatus comprising a rigid mold flask is adapted to contain a mold pattern and sand and when containing sand, has a combined center of gravity. A support is provided for resiliently supporting the mold flask for horizontal movement only of the mold flask as a whole body, and a vibrator shakes the mold flask to provide horizontal vibrational forces directed in a horizontal plane extending approximately through the combined center of gravity. This patent discloses the structural details associated with the horizontal vibration means referred to in the above-mentioned U.S. Pat. No. 4,593,739.
Although the systems illustrated in these patents provide certain improvements over older techniques, they are limited in the flexibility and control of the direction and magnitudes of the net vibrational force vectors which they can produce. Thus, they cannot take full advantage of the potential flexibility and degree of intricacy that the evaporative pattern process is capable of achieving.